The present invention concerns an eddy current test coil assembly having at least one excitation winding and at least one sensing winding, each comprising only one or two turns. Eddy current test coils are used to determine the presence of flaws in metal workpieces and to ascertain certain properties of such workpieces.
A test coil assembly of the above type is described, for example, in German patent application DT - OS No. 2,115,247. The coil assembly described therein includes a single turn excitation winding surrounding a tubular workpiecee to be tested and two sensing coils, disposed coaxially on each side of the excitation winding and connected differentially. The advantages which result from the use of single turns are self-evident, namely, simplicity of construction, minimum electrical insulation, low space requirement together with high resolution capability and a short non-tested tube end, made possible because the turns differentially coupled can be disposed in close proximity. There is, however, a further significant advantage. When using eddy current test coils, specifically when small defects are to be located or readings of low amplitude are to be made, it is most important to dispose the sensing winding as close as possible to the surface of the workpiece under test. This is particularly difficult when irregular contours are encountered, e.g. tubes of complex shape. In such a case the test coil winding has to conform to the contour of the workpiece. An arrangement of that type usually is possible only at considerable expense and often can be provided only in an incomplete manner, typically, in the case of surfaces exhibiting indentations. Using single turns, however, it is easy to conform the winding to the surface of any workpiece under test.
Aside from the above stated advantages, the use of single turns for test coils has also significant disadvantages, which to date have prevented this type of coil from being adopted more universally. The high currents required for excitation, lead to contact problems at the plug-in connections of the input cables. The usually low signal amplitude on the sensing portion permits high signal interference from stray fields, particularly those arising in the cable. This, in turn, causes the signal to noise ratio to deteriorate to an unacceptable level. Since it is necessary to match the amplifier input to the low-resistance sensing winding, difficulties also occur on the sensing side resulting from the spontaneous changes in the contact resistance of the plug-in connections. A further aspect concerns the extent to which the differentially coupled turns can be balanced for achieving a minimum residual carrier voltage. With conventional test coils having a large number of turns, it was always easy to effect precise balancing by adding or removing a number of turns on one of the two differential windings. Quite naturally, this physical change cannot be done on single turn coils. In addition, individual turns, according to the patent application referred to above, have to be constructed extremely ruggedly to provide the requisite mechanical strength. This, however, reduces considerably the possibility of measuring eddy current fields immediately above the workpiece surface.